1. Field of the Invention
The present invention relates to an alkaline storage battery such as nickel-hydrogen storage battery and nickel-cadmium storage battery. More particularly, the present invention relates to an improvement in the positive electrode active material mainly composed of nickel hydroxide to be incorporated in non-sintered nickel electrode and the process for the production thereof.
2. Description of the Related Art
A nickel electrode which is normally used as a positive electrode for this kind of alkaline storage battery undergoes a reversible reaction involving the conversion of divalent nickel hydroxide (Ni(OH)2) to trivalent nickel oxyhydroxide (NiOOH) as represented by the following equation (1) during charging and the conversion of trivalent nickel oxyhydroxide (NiOOH) to divalent nickel hydroxide (Ni(OH)2) as represented by the following equation (2) during discharging.
Ni(OH)2+OHxe2x88x92xe2x86x92NiOOH+H2O+exe2x88x92xe2x80x83xe2x80x83(1)
Ni(OH)2+OHxe2x88x92xe2x86x92NiOOH+H2O+exe2x88x92xe2x80x83xe2x80x83(2)
This reaction is not a complete reversible reaction. Thus, during the process of reconversion of nickel oxyhydroxide (NiOOH) to nickel hydroxide (Ni(OH)2) by discharging, the discharging reaction stops when the valence of nickel hydroxide thus produced is about 2.2. Accordingly, electricity according to a valence of 0.2 always remains in the negative electrode as an irreversible content. The remaining electricity makes no contribution to the capacity of the battery.
Methods for eliminating the irreversible capacity in the negative electrode have been proposed in Patent No. 2765008, JP-A-10-74512 (The term xe2x80x9cJP-Axe2x80x9d as used herein means an xe2x80x9cunexamined published Japanese patent applicationxe2x80x9d), etc. The method proposed in Patent No. 2765008 involves the use of, as a positive electrode active material, a nickel oxide having a valence of greater than 2 obtained by partly subjecting nickel hydroxide to chemical oxidation. In this manner, the negative electrode has no electricity left behind as an irreversible content. Thus, all the electricity makes contribution to the capacity of the battery.
On the other hand, the method proposed in JP-A-10-74512 involves the filling of a porous metal material with nickel oxyhydroxide particles coated with cobalt oxyhydroxide or cobalt hydroxide or a solid solution mainly composed of nickel oxyhydroxide as an active material. In this manner, the coating of nickel hydroxide particles as active material with cobalt oxyhydroxide as an electrically-conducting agent makes it possible to provide firm and uniform physical disposition of electrically-conducting agent as well as improve the physical packing of the active material as compared with the use of particulate cobalt hydroxide as an electrically-conducting agent. Further, the resulting positive electrode can be kept partly oxidized. Accordingly, a battery having a reduced irreversible electricity can be formed.
However, the positive electrode active material obtained by oxidizing nickel hydroxide to enhance the order thereof and give a higher order nickel hydroxide or the positive electrode prepared with such a positive electrode active material is disadvantageous in that when stored in the air, it undergoes a reaction represented by the following equation (3) to cause the higher order nickel hydroxide to be reduced to nickel hydroxide (Ni(OH)2), making it difficult to store over an extended period of time.
NiOOH+1/2H2Oxe2x86x92Ni(OH)2+1/4O2xe2x80x83xe2x80x83(3)
Further, an alkaline storage battery comprising a positive electrode plate prepared with a positive electrode active material obtained by oxidizing nickel hydroxide to higher order nickel hydroxide allows the positive electrode to undergo self-discharging when the positive electrode active material reacts with an electrolytic solution during storage. During this self-discharging, the negative electrode tries to discharge by the same capacity. However, the negative electrode has no electricity and thus oxidizes itself instead of discharging. Thus, an oxide layer is formed on the surface of the negative electrode, passivating the negative electrode. As a result, the capacity of the negative electrode is lowered, making it impossible to obtain desired properties.
Therefore, the present invention has been worked out to eliminate the foregoing problems. An object of the present invention is to provide a high capacity alkaline storage battery having a reduced excess capacity of negative electrode by making nickel hydroxide of raised order (higher order nickel hydroxide) or nickel positive electrode comprising this higher order nickel hydroxide as a positive electrode active material storable over an extended period of time while inhibiting self-discharging of the nickel positive electrode. In order to solve the foregoing problems, the alkaline storage battery according to the invention comprises a positive electrode active material mainly composed of nickel hydroxide, characterized in that the nickel hydroxide is a higher order nickel hydroxide covered with a cobalt compound on the surface thereof or in the vicinity thereof and the higher order nickel hydroxide is provided with at least one compound selected from the group consisting of yttrium compound, erbium compound and ytterbium compound on the surface thereof or in the vicinity thereof.
The use of such a higher order nickel hydroxide as a positive electrode active material makes it possible to reduce excess capacity of negative electrode. Further, since the surface of the higher order nickel hydroxide is coated with a cobalt compound, a good electrically-conductive network can be formed in the positive electrode to enhance the percent use of active material, making it possible to obtain a high capacity storage battery. When the higher order nickel hydroxide coated with a cobalt compound on the surface thereof is provided with at least one compound selected from the group consisting of yttrium compound, erbium compound and ytterbium compound on the surface thereof or in the vicinity thereof, the higher order nickel hydroxide can be prevented from discharging itself because these additive compounds can enhance the oxygen production potential. As a result, the higher order nickel hydroxide can be kept stable during prolonged storage. Further, even a battery assembled with this active material can be prevented from discharging itself, making it possible to obtain a high capacity alkaline storage battery having a reduced excess capacity of negative electrode.
When the cobalt compound with which the surface of the higher order nickel hydroxide is coated is a high order cobalt compound containing alkaline cations, there is no boundary between the high order cobalt compound formed on the surface of the higher order nickel hydroxide and the higher order nickel hydroxide as core, giving firm bond between nickel and cobalt that enhances the mechanical strength of active material particles and lowers the electrical resistance between nickel and cobalt. The resulting high rate discharge capacity is raised. The presence of alkaline cations prevents the cobalt compound from being oxidized by an oxidizing agent as well as by water, making it possible to secure stability of the cobalt compound and enhance the oxygen production potential and hence further inhibit the self-discharging after storage.
When the average valence of the higher order nickel hydroxide is lower than 2.1, the irreversible capacity of the negative electrode cannot be sufficiently eliminated, making it impossible to effectively utilize the inner space of the battery. On the contrary, when the average valence of the higher order nickel hydroxide is higher than 2.3, the dischargeability of the negative electrode is lowered. Accordingly, the average valence of the higher order nickel hydroxide is preferably from 2.10 to 2.30.
Preferably, the positive electrode active material comprises a higher order nickel hydroxide of complex particles covered with a cobalt compound on the surface thereof and said higher order nickel hydroxide is provided with at least one compound selected from the group consisting of yttrium compound, erbium compound and ytterbium compound on the surface thereof or in the vicinity thereof.
Preferably, the higher order cobalt compound is immersed into the higher order nickel oxide particles and boundary between said higher order cobalt compound and the higher order nickel oxide particles is diminished and composition changes successively.
Preferably, the higher order cobalt compound is localized on a surface of said higher order nickel oxide particles.
Preferably, said positive electrode active material further comprises zinc oxide in soluble state.
Preferably, the positive electrode active material is filled with a porous metal plate as a current collecting plate to form a positive electrode.
Preferably, the positive electrode active material and conductive particles are formed to be positive electrode.
The process for the production of an alkaline storage battery according to the invention comprises a retaining step of allowing a cobalt compound and at least one compound selected from the group consisting of yttrium compound, erbium compound and ytterbium compound to be retained on the surface of the nickel hydroxide or in the vicinity thereof, and an order-enhancing step of enhancing the order of the nickel hydroxide. Alternatively, the process for the production of an alkaline storage battery according to the invention comprises a retaining step of allowing a cobalt compound to be retained on the surface of the nickel hydroxide or in the vicinity thereof, an order-enhancing step of enhancing the order of the nickel hydroxide, and an attachment step of allowing at least one compound selected from the group consisting of yttrium compound, erbium compound and ytterbium compound to be attached to the higher order nickel hydroxide on the surface thereof or in the vicinity thereof.
The provision of the foregoing various steps makes it possible to coat the higher order nickel hydroxide with a cobalt compound on the surface thereof or in the vicinity thereof and provide the higher order nickel hydroxide with at least one compound selected from the group consisting of yttrium compound, erbium compound and ytterbium compound on the surface thereof or in the vicinity thereof. In this arrangement, excess capacity of negative electrode can be eliminated. Further, a good electrically-conductive network can be formed in the positive electrode to enhance the percent use of active material, making it possible to obtain a high capacity storage battery. Further, the higher order nickel hydroxide can be kept stable even over an extended period of time. Moreover, even a battery assembled with this active material can be prevented from discharging itself, making it possible to obtain a high capacity alkaline storage battery having a reduced excess capacity of negative electrode.
The oxidation of nickel hydroxide with an oxidizing agent at the order-enhancing step makes it possible to enhance the order of nickel hydroxide by merely adding a simple step of adjusting the supplied amount of the oxidizing agent. In this manner, this kind of battery can be easily produced.
By mixing nickel hydroxide with a cobalt compound or coating nickel hydroxide with a cobalt compound at the retaining step, and then subjecting the mixture or coated material to heat treatment in the presence of an alkaline aqueous solution and oxygen so that the cobalt compound is enhanced to higher order, a good electrically-conductive high order cobalt layer can be formed on the surface of the higher order nickel hydroxide, making it possible to form a firm electrically-conductive network in the positive electrode. In this arrangement, the high rate discharge capacity is enhanced. Further, since the high order cobalt layer contains alkaline cations, the oxidation of the cobalt compound by the oxidizing agent as well as by water can be inhibited, making it possible to secure the stability of the cobalt compound and enhance the oxygen production potential and hence further inhibit self-discharging after storage.